Signal blocking and fading are constant concerns in communication systems, and especially in cellular communication systems. Signal blocking occurs when an obstruction (such as a building) impedes the reception of a communication signal. Fading occurs due to somewhat random attenuation changes within the transmission medium. Fading can also occur due to an obstruction (such as a building). In the field of cellular communications, signal blocking and fading can lead to poor quality communications, dropped calls, and customer dissatisfaction.
In conventional cellular systems, efforts are taken to locate cell sites, adjust power levels, adjust antenna structures, and allocate channels to cells so that signal blocking and fading are minimized. Once cell site locations, power levels, antenna structures, and channel allocations are established for conventional systems, they tend to remain relatively static so that any changes are difficult to make. Therefore, some blocking is inevitable.
When cellular base stations (e.g., satellites) are located in orbit around the earth, the blocking and fading problems become more complicated. In order to reduce cell size and reuse distance, base station satellites are desirably placed in low earth orbits, within which they travel at a high rate of speed. This may cause the cells to move over the surface of earth, making signal blocking and fading a problem that changes with time. Cell site location is not fixed but rather changes with time. In addition, antenna patterns change as a function of time and location. Consequently, blocking and fading problems are not as predictable in a satellite communication system as they are in conventional cellular systems.
Satellite communication systems need an improved means for mitigating signal fading and signal blocking. Because of the cost associated with launching and maintaining satellites in space, a satellite communication system must take full advantage of the resources on-board the satellites. Also, a method and apparatus are needed which accomplishes this by making modifications on the terrestrial side of the system. What is further needed is a means for bringing larger amounts of communication capacity to bear on densely populated areas while using only a limited amount of the radio frequency spectrum.